The present invention is generally in the field of wireless communication. More specifically, the invention is in the field of wireless communication receivers using interference removal.
Wireless communication systems include transmitters capable of transmitting radio frequency (rf) signals to receivers. Specifically, transmitters transmit a desired rf signal and receivers receive a received rf signal comprising the desired rf signal, noise and interference. Noise is defined as non-local (with respect to the receiver) unwanted rf signals, which can be caused by, for example, biological sonar, rf jammers and solar flares. Interference is defined as local (with respect to the receiver) unwanted rf signals, which can be caused by, for example, electromagnetic radiation from electrical equipment and power lines in close proximity to receivers. Wireless communication systems attempt to improve reception quality by eliminating or reducing interference.
Hardware notch filtering is a typical method for reducing narrowband interference in wireless communication system receivers. Hardware notch filtering uses an external reference signal to create notch filters notched at the fundamental and harmonic frequencies of the external reference signal. The hardware notch filters operate in the time domain to remove or reduce narrowband interference from received rf signals. Disadvantageously, hardware notch filtering requires external references, cannot remove strong narrowband interference and can require multiple notch filters.
Typical excision devices attempt to reduce narrowband interference in wireless communication system receivers by operating in the frequency domain, which does not require external references. Typical excision devices include a transformer, a power estimator, an excisor and an inverse transformer. The transformer performs “windowing” (e.g., Blackman windowing) and the well-known Fast-Fourier Transform (FFT) on a received signal to transform the received signal from the time domain to the frequency domain. Windowing techniques are described in more detail in an article published by the Proceedings of the IEEE in 1978, entitled “On the Use of Windows for Harmonic Analysis with the Discrete Fourier Transform”, authored by Frederic J. Harris, which is hereby incorporated by reference in its entirety for its teachings on windowing techniques. The FFT is described in more detail in a book published by Macmillan in 1988, entitled “Introduction to Digital Signal Processing”, authored by J. G. Proakis et al., which is hereby incorporated by reference in its entirety for its teachings on the FFT. The power estimator receives frequency domain data from the transformer and determines a power vector. The excisor receives frequency domain data from the transformer and the power vector from the power estimator. The excisor removes narrowband interference by determining whether the power vector exceeds an excision threshold. The inverse transformer receives frequency domain data from the excisor and inverse transforms the data into time domain data. Disadvantageously, typical excision devices cannot adapt to noisy changes in environment. For example, high impulsive noise (i.e., short duration, high-amplitude noise) and high interference precludes typical excision devices from removing narrowband interference in Extremely Low Frequency (ELF) communications.
Therefore, a need exists for narrowband interference excision devices that do not require an external reference for narrowband interference. In addition, a need exists for a method and apparatus for narrowband interference excision devices that operates in high impulsive noise and high interference environments.